Switch mode power supply is widely employed in modern electrical products. Traditional switch mode power supply may apply pulse width modulation (PWM) control method to regulate the output of the power supply, which usually involves in complicated structure and slow transient response. Therefore, its performance in high current applications is limited.
Compared with PWM control method, pulse frequency modulation (PFM) control method, which primarily comprises constant on-time control method, constant off-time control method and hysteresis control method, possesses advantages in transient response performance and circuit design. FIG. 1 illustrates a prior art constant on-time converter circuit 10. Constant on-time converter circuit 10 primarily comprises a direct current to direct current (DC-DC) buck converter 101, and a controller 102. In the controller 102, an error signal VEAO is compared with a ramp signal VRAMP by a comparator 105. Wherein, VEAO is generated by an operational transconductance amplifier 104 according to a difference between a reference signal VREF and an output voltage feedback signal VFB. When the ramp signal VRAMP falls lower than the error signal VEAO, a pulse is provided to an on-timer 103 so that on-timer 103 turns a primary switch M1 on and turns a synchronous switch M2 off. After a constant period T1 passes, on-timer 103 turns off the primary switch M1 and turns on the synchronous switch M2. With the complementary actions of switches M1 and M2, an output voltage VOUT is generated from an input voltage VIN.
When a transient load step occurs, the error signal VEAO may react smoothly. To achieve fast transient response for the converter circuit 10, the peak to peak amplitude of the ramp signal VRAMP should be very small. But the small ramp amplitude may cause two major problems. First, it means that the noise margin of the converter circuit 10 is relatively small. When the converter circuit 10 is operating in a noisy operation environment, this feature may lead to a big risk. Second, to cope with small amplitude signal, circuit elements such as comparators need draw more power to achieve desired speed.
The use of the same reference label in different drawings indicates the same or like components.